1. Field of the Invention
The present general inventive concept relates generally to a color image forming apparatus capable of correcting an error between scanning lines for each color due to an alignment error between laser scanning units, and a control method of the color image apparatus.
2. Description of the Related Art
As illustrated in FIG. 1, a conventional color image forming apparatus includes a paper feeding unit 10, laser scanning units 20K, 20C, 20M and 20Y, developing cartridges 30K, 30C, 30M and 30Y, photoconductive drums 31K, 31C, 31M and 31Y, a transfer unit 40, a fixing unit 50, and a paper discharge unit 60. In these conventional laser scanning units, the developing cartridge and the photoconductive drum are typically individually provided for each color.
If a printing instruction is input to the color image forming apparatus, images for each color are formed on the photoconductive drums 31K, 31C, 31M and 31Y by laser beams radiated from the laser scanning units 20K, 20C, 20M and 20Y for each color, respectively. Once the images are formed, a paper P fed from the feeding unit 10 passes between a paper transfer belt 41 and each of the photoconductive drums 31K, 31C, 31M and 31Y. At this time, the images for each color respectively formed on the photoconductive drums 31K, 31C, 31M and 31Y are transferred onto the paper P by respective corresponding transfer rollers while being overlapped with each other. The paper P, onto which the images are transferred, passes through the fixing unit 50 for applying high-temperature heat and pressure such that the images are fixed on the paper. The paper is then discharged to the outside through the paper discharge unit 60.
The colors of toners used in a developer are yellow Y, magenta M, cyan C and black K. The respective four colors are transferred to overlap with one another, thereby forming a complete image. In order to provide an image with a high image quality, unit images formed by the respective colors should exactly overlap with one another.
In a case where a plurality of laser scanning units are provided corresponding to the different colors of toners, and where the scanning positions of laser beams radiated from the laser scanning units are not registered for each color, unit images for each color do not exactly overlap with each other.
Laser scanning units corresponding to the different colors may deviate from one another when they are mounted. Further, even though each laser scanning unit is mounted at a right position, the laser scanning units may deviate from one another due to vibrations generated when driving a polygonal mirror for deflecting a laser beam during use of the laser scanning unit.
For example, FIG. 2 illustrates a case where laser scanning units 20C and 20M for respectively scanning laser beams for cyan and magenta deviate from each other at a predetermined angle A. Here, the scanning positions of scanning lines of laser beams respectively deflected by rotatably driven polygon mirrors 21C and 21M inside the upper and lower laser scanning units 20C and 20M are misaligned, causing the scanning lines scanned on the surfaces 25C and 25M of the photoconductive drums 31C and 31M corresponding to cyan and magenta through fθ lenses 22C and 22M to be different from each other.
In a conventional image forming apparatus, whenever a laser beam of the corresponding color image is scanned for a line from each of the laser scanning units 20C and 20M, a horizontal synchronization signal is output. As illustrated in FIG. 3A, such horizontal synchronization signals Hsync[C] and Hsync[M] for each color is reflected by reflection mirrors 23C and 23M, and then received by optical sensors 24C and 24M, respectively. The horizontal synchronization signals Hsync[C] and Hsync[M] are synchronized with each other. However, as illustrated in FIG. 3B, the scanning position and length of scanning lines Line[C] and Line [M] respectively scanned on the photoconductive drums 31C and 31M may not correspond to each other due to a position error between laser scanning units 20C and 20M.
Conventionally, an auto color registration is performed to solve such a position error. A method is applied, in which a difference between positions of scanning lines for each color in accordance with the result of the performance of the auto color registration is detected, and horizontal synchronization signals for each color is then corrected in order to correct the difference between the positions as illustrated in FIG. 4A. As a result, scanning lines for each color respectively scanned photoconductive drums can be set to correspond to each other in accordance with the corrected horizontal synchronization signals as illustrated in FIG. 4B.
However, according to the conventional method, a color registration operation should be performed whenever a printing operation is performed for a certain time, and a process of printing a test pattern on a paper, scanning the test pattern, and then analyzing the test pattern is typically carried out to perform the operation. For this reason, there is a problem in that a load of signal processing for the analysis of the test pattern is weighted.
In order to solve such a problem, a method has been proposed in Japanese Unexamined Patent Publication No. 11-55472, in which a termination detecting sensor is additionally mounted at a scanning termination region in addition to a synchronization detecting sensor for detecting horizontal synchronization signals. In this method, a laser beam scanning time is counted from a time point when a horizontal synchronization signal is detected by the synchronization detecting sensor to a time point when a termination signal is detected by the termination detecting sensor, and an image data read timing is shifted according to the result obtained by comparing the counted scanning time with a reference value, thereby correcting an error between scanning lines for each color.
The method according to the above document may be simpler than the conventional color registration operations discussed previously above. However, one drawback of the method proposed in JP Pub. No. 11-55472 is that a reference clock with a high frequency should be used to count a scanning time from a synchronization detection time to a scanning termination time whenever a line is scanned, with results being that a process of storing information counted using such a reference clock is required. Accordingly, additional components for implementing such a method are required. Moreover, the problem related to a weighted load for signal processing still remains.
Further, the method proposed in JP Pub. No. 11-55472 is also limited in that a termination detecting sensor should be provided separately from the synchronization detecting sensor, thus requiring separate sensors to detect the synchronization and termination signals, respectively.